The invention relates to tape speed override operation of a helical video tape recorder and, in particular, to a tape speed override system for faithfully recovering helically recorded digital audio signals via helical audio processing channels, while also recovering associated video signals.
In the television industry, and in the course of broadcasting pre-recorded movies, newscasts, commentaries and, particularly, advertisements of varying lengths, it is common for a television broadcast station to have occasions when it is highly desirable to be able to broadcast a recording in slightly less time, or slightly more time, than the length of the recording. For example, it may be desirable to broadcast a recorded tape having a length of 11 minutes in a time slot of 10 minutes, or in a time slot of 12 minutes. It is equally desirable to transmit the recording in its entirety, without omitting or repeating, or otherwise editing, any of its contents. The preferred manner for performing such a process has been to time compress or expand the playback of the recording and thus of the recorded material, generally by a playback process known in the field of television as tape speed override (TSO). As implied, the tape speed override mode of operation means that the tape is driven at a speed which is a selected percentage greater or less than normal play speed.
In the past, on video tape recorders with longitudinal audio channels such as analog helical recorders, time compression or expansion of recorded material, that is, TSO, were accomplished by varying the linear tape speed according to the desired degree of compression or expansion. In such recorders, the helically recorded video information is compressed or expanded by skipping or repeating fields or frames of video as required to match the average speed of the playback process. It follows that the associated audio channels on the longitudinal tracks simply are played back at the correspondingly faster or slower speed of the video tape.
However, in a rapidly developing field of, for example, digital video/audio recording/reproducing, very sophisticated digital video recorders employ a helically recorded audio format, wherein audio signals are recovered, along with video signals, via associated helical audio channels and the respective helically scanning, audio/video heads. Precise recovery of the audio presents significantly worse problems when dealing with such a helical audio format, since helically scanning heads as a rule, may be used to skip or repeat tracks of video in order to perform video special effects such as stop, slow motion, fast motion, etc. However, as is well known, audio must be continuously recovered since any interruption of the digital audio data will cause very objectionable pops and noise. Accordingly, the respective heads which also are tracking the helical audio cannot skip or repeat with the video since this would interrupt the digital audio data. This condition complicates the process of time compression or expansion as when using the process of tape speed override when reading out helical audio.
One solution to such a problem of recovering the helical audio and video signals, is to run the entire playback process off-speed by speeding up, or slowing down, the reference clock for the entire transport and signal processing system, thus compressing or expanding the recovered audio at the outputs of the helical audio channels, while also recovering the video. The processing systems used with such an off-speed reference clock include both the video TBC and an audio TBC, which can be clocked at the non-normal rate of the off speed reference clock.
However, this causes problems in the video signal processing circuit, and particularly at the recorder output, wherein the video timing pulses such as horizontal sync and color burst are now not at the correct rates, causing the video signal to be unusable by other equipment.
In this off-speed system however, the helical scanner also must be phase and frequency locked to the changing tape phase and frequency. In addition, the system also must be robust enough to handle all the other variables typical of such helical video tape recorders. These variables include interchange capabilities, temperature and humidity effects, track straightness and interchange insert edits. Typically, in video tape recorders with controllable scan tracking heads, a dither effect is added to the tracking elevation of the scan heads to find and keep the optimium scan position on the track. This technique solves most of the small tracking problems of previous mention.
To overcome the problem of the output video timing pulses being at the wrong frequency, it would be necessary to run the scanner and TBC with the correct television reference while the tape linear speed is at a non-normal rate. But this will cause a break in the audio signal when the tracking head is forced to jump. A jump is required to compensate for the elevation change due to a change in the linear tape position relative to the reference phase-locked scanner. However, if the scanner is unlocked and run at a frequency proportional to the tape speed, the elevation will not change and head jumping is not required. Thus, the audio signal may be contiguously recovered, which is the ultimately desired result.
In the helical audio/video tape recorders of previous discussion, it is thus highly desirable to run the transport from a variable reference and the video signal processing circuits at a constant standard synchronous reference, and to allow the video TBC and the controllable scan tracking heads to operate in conventional fashion as when performing in the normal play mode. In addition, it is equally desirable to then run the audio signal processing circuits fast or slow as required, while keeping the audio circuits on a reference clock which is running at the desired TSO rate. In this way, the audio can be loaded at a faster or slower rate, and read out at an equally faster or slower rate, to provide TSO without causing problems in the video signal processing.
An immediate problem caused by using a constant reference in the TSO mode is that the recorder system now must feed more or less data into and out of the system. However, it must be remembered that the scan tracking heads cannot be allowed to jump, since they also are reading out audio signals which cannot be interrupted or repeated. Accordingly, if more audio and video data are to be handled, the transport speed must be increased, which causes its phase to be changed proportionaly. As a result, as previously mentioned, the scanner breaks lock and its phase will wander relative to the phase of the control track, that is, relative to the tape speed, if the tape speed is not changed.
The present invention overcomes the problems and disadvantages in the helical audio system of previous mention, when operating in the TSO mode, while allowing a range of TSO operation generally of the order of .+-.15%, but which could be larger. More particularly, the invention uses a constant standard reference clock to run the transport and video signal processing systems, to thereby alleviate the problems of previous mention associated with non-normal video signal processing. In addition, the tape speed is increased or decreased to provide the desired TSO rate while the scanner speed is increased or decreased proportionately to keep the scanner and tape in phase-locked condition to prevent head jumping of tracks. Given the example of TSO of faster than normal rate, for example, 10% or 1.1 faster, the invention slowly increases the speed of the scanner. In order to maintain phase lock, the speed of the tape and thus of the control track, is increased proportionately. Since more audio data now is being recovered from tape, there is the risk of overrunning the audio TBC, thereby spilling bits of audio data and causing an objectionable break in the audio signal.
To circumvent the overrunning of the audio TBC, the rate of the audio clock is increased to thereby increase the readout of data from the TBC. As the TBC contents drain to a preselected level, for example, one-half of the contents as represented by the center of a TBC time window or range, the continued drain is in effect "detected", and an error or correction signal is generated. The scanner speed then is increased in response to the correction signal indicative of the TBC centering. The tape position, or control track phase, is locked to the scanner angular position or phase. As the scanner velocity increases commensurate with the desired TSO rate, the tape speed is increased by means of control track and capstan servo loops, and the system stays in lock in the TSO mode without head jump or loss of any audio data. Meanwhile, the video signal processing is performed in conventional fashion with suitable skipping of pertinent fields or frames via a frame store to provide the video data compression required to match the audio data compression.
It may be seen that the present invention employs a concept of synchronized cueing such as described in copending U.S. patent application SN-036,630, filed Apr. 10, 1987, Apparatus And Method assigned to the same assignee as this application. The subject matter of the prior application is incorporated herein by reference. In the copending application, the system synchronizes to a linearly moving position (a moving cue location) and thus is concerned with a velocity change Conceptually, in the TSO mode of this application, the system synchronizes to a moving phase, that is, synchronizes to phase instead of position as in the prior application, and herein is concerned with phase change in addition to velocity. To this end, the invention utilizes, in effect, the contents level or time window center of the audio TBC to derive an error or control signal indicative of the positional error of the scanner relative to the audio data, and then derives another signal indicative of the positional error of the control track relative to the scanner position. The current phase of the scanner is predicted, and the resulting signal is applied as a positional error to correct the speed of the tape via the capstan servo. The TSO nominal velocity of the scanner is known, since it is the desired TSO rate, and thus is the corresponding audio data rate desired at the video tape recorder system output.
By way of example, the invention includes an audio TBC in a video tape recorder (VTR) with a helical audio format, and a scanner servo with an associated scanner and tach. The TBC normally is clocked with an audio clock signal of 48 kiloHertz(KHz) derived from an 18 megaHertz(MHz) audio clock generator. However, the nominal frequency of the 48 KHz signal is modified slightly during TSO via a TSO rate signal commensurate with the desired data compression or expansion. A CPU is supplied with a digital number indicative of the desired scanner operation and, in turn, supplies a phase comparator with a phase error signal indicative of a scanner speed required to maintain a selected data content in the TBC. The phase comparator also receives a scanner tach phase signal from the scanner. The comparator supplies the result of the comparison of the signals to the scanner servo, which then drives the scanner. The scanner tach also supplies the scanner tach phase signal to a control track servo, which also receives a control track signal from a control track head in generally conventional servo arrangement. The control track servo is coupled to a capstan servo which drives a capstan and its associated capstan tach to maintain a selected, constant, tape to scanner phase relationship to thus prevent a scan head from jumping tracks. A capstan tach signal is supplied as feedback to a second input of the capstan servo in generally conventional servo arrangement.
It may be seen that the scanner is responsive to the error signal from the CPU commensurate with the audio TBC contents, and the capstan servo and control track servo are responsive, in turn, to the scanner tach signal to maintain a phase-locked condition therebetween. Thus, contrary to normal VTR operation, in the TSO mode of operation in accordance with the preferred embodiment of the invention, the conventional functions of the scanner servo and of the control track and capstan servos, relative to the audio signal playback processing circuits, are reversed.